A. Technical Field
This invention relates to phase-locked loop (“PLL”) systems, and more specifically, to the removal of jitter in the PLL output during the synthesis of certain clock signals in PLL systems.
B. Background of the Invention
Digital communication systems are now widespread, providing data conduits for numerous data types being transmitted from a source to a client over a network comprising one or more of these transmitter/receiver links or nodes. In order to accurately reconstruct the transmitted data at the client end, it is desirable to reproduce the client signal clock; the original data clock supplied to the network at the source end of the transmission link. In this way, time based data will be preserved at the client end. For example, if voice service is being transmitted, the signal can be spliced back together in a time-based cohesive manner with the use of an extracted client signal clock such that no dropouts occur at the client end. Other transmitted forms of data types which utilize an extracted client signal clock at the receiving client end include, but are not limited to, compressed voice technology, facsimile transmission, digital video transmission, and other quality of service based data types.
In the prior art, phase-locked loop (“PLL”) systems are used to extract the desired client signal clock. Turning to FIG. 1, a conventional PLL system 100 is shown. The purpose of the PLL system 100 is to provide an output clock frequency 160 which is proportional to an input reference clock frequency 110. As the input reference clock frequency 110 changes, the PLL 100 will track the change such that the output clock frequency 160 changes proportionally to the input reference clock 110.
A second order conventional PLL system includes a phase detector 120, a loop filter 130 and a voltage controlled oscillator 140 (“VCO”). The output fout of the VCO 140 provides feedback to the phase frequency detector 120 or comparator, as part of the PLL system, and is compared with an input reference signal fref 110 by the phase detector 120, which results in an error signal. The error signal is representative of the phase or frequency difference between the two signals, fout and fref. The error signal is then feed to the loop filter 130 via one of two signals, +fvco or −fvco. For example, if the proportional frequency of the output signal fout is lagging the input reference signal fref, then the error signal +fvco is provided to the VCO 140 to command the VCO 140 to increase the output frequency of fout to track, or otherwise proportionally change with respect to, the input reference signal fref. The loop filter 130 is a low pass filter which filters out higher frequencies and provides at its output a frequency control signal to the VCO 140.
In many applications, it is undesirable to have the input reference signal fref and the output signal fout at the same frequency and, thus, the signals are scaled. As shown, the feedback signal fout is scaled by a factor M 150 and the input reference signal fref is scaled by a factor of N 115. This results in the following relationship between the output signal fout and the input signal fref:
                              f          out                =                              M            N                    ·                      f            ref                                              (        1        )            
A problem with the use of the above relationship (1) in conventional PLL systems in the extraction of the end client signal clock is that they are susceptible to large changes in the input reference signal fref. A conventional PLL as described herein is sensitive to sudden changes in the reference signal fref resulting in excessive frequency and phase variations which can cause the end terminating client receiver to slip bits. Such fast changes cannot be adequately filtered out resulting in jitter or wander at the output signal fout. If severe, such jitter or wander can cause end receivers to lose lock on the client signal, resulting in dropouts, apparent in intermediate audible clicks in voice service data for example.
Under certain circumstances, delivering specific types of payloads one can use the justification count (“JC”) of a payload digital wrapper to correct for excessive frequency and phase variations. AMCC or G.709 specifications, for example, constrain the JC value to +/−1, since such systems only support +/−1 JC. This may not lead to an undesirable jitter problem. In the client receiving end node the plus or minus one clock represented by the JC value can be interpolated over an entire frame. Since each frame of data is thousands of bytes in length, the frequency shifting of one clock cycle over the entire frame by the PLL system will result in minimal jitter.
However, one problem in the foregoing scheme is that the resulting system is limited in use, being able to adequately transmit payloads of certain configurations, where the JC is +/−1 for example, while not being suitable for the transmission of other payloads. Furthermore, the foregoing scheme offers little scalability with regards to newer network configurations relying on new data frame formats which may require justification count values in the thirties or higher.